That switch have 2 vacuum lines to it? You either have something stuck in your fresh air intake, or something is wrong on the venting side. Check for birds/nests/leaves etc in both intake and exhaust venting.
Not the biggest I've seen, but I'm surprised that it was still running. Usually by then it's tripping the roll out switch. (unless your pressure switch was in series with the roll out)I just had this exact same issue with my furnace. Furnace would run great for a while and then random pressure switch shut offs. I had a fairly large hole ...about 2" in my heat exchanger and none of my C02 alarms ever went off. I think because I have a draft inducer. My advice...open up the furnace and inspect for a crack or whole...not worth the risk.